In order to manufacture a rotor by welding a number of elements together, the elements are forged and machined first.
These elements have a cylindrical shape with inner cavities (through cavities), and are provided at the faces to be welded with collars that define between adjacent elements (when two or more elements are joined together) circumferential slots.
Then a welding is realised within the slots completely filling them in, while at the same time melting the collars and eventually also providing additional material, which is illustrated by US2008/0211339, the contents of which are incorporated by reference.
An improvement to this process includes vertically stacking all the elements to be welded one above the other to form a pile, with the collars and all the cavities connected together. Then a first welding (pre-welding) is realised within the slots (from the bottom to the top of the pile) to connect the elements together; this welding is similar to the one disclosed in US 2008/0211339, but it is such that it does not fill the whole slots in.
The pile is then tilted in the horizontal position and a final welding is realised, completely filling the slots in. Then the pile is tilted in the vertical position again and thermal treatments are carried out. Finally, the pile is tilted again in the horizontal position for a final machining, which is illustrated in WO 2009/065739, the contents of which are incorporated by reference.
In order to avoid oxidation during welding, before the welding process starts, the mutually connected cavities are purged with an inert gas or mixture of gases (such as for example a mixture of H2 and Ag or H2 and N2).
In this respect holes are provided at the sides of the collars, to inject the purge gas or mixture (these holes are also used to monitor the welding process from the inner portion of the cavities).
Nevertheless, since the pile is vertical, and since the oxygen is heavier than the inert gas or mixture typically used, there is the risk that the welding at the lower part of the pile is realised in presence of oxygen. This must be avoided.